High on the Beartooth Plateau, YERC researchers studying the rare mountain fox population there observed never-before-seen behavior that may help to explain the population's persistence in such an extreme environment: the adaptable foxes were raiding red squirrel middens for their whitebark pine seeds. But whitebark pines in the Greater Yellowstone Ecosystem face threats on multiple fronts; so what does that mean for these native foxes? As part of our efforts to collaborate with other groups working on important issues affecting the GYE, we provided the following cover story for the Whitebark Pine Ecosystem Foundation's newsletter, Nutcracker Notes.
The Slyest Seed Predator: Interactions Between Red Fox
and Whitebark Pine in the Greater Yellowstone Ecosystem
By Patrick Cross, Yellowstone Ecological Research Center
Originally appeared in Nutcracker Notes, Summer
2015
"Aha, there's a fox scat for you, Joel!" I
proclaimed, pointing ahead with my ski pole across the otherwise unblemished
snow.
It was a crisp January day, with alpine sunshine sparkling
off ice crystals suspended in the mountain air, high on the Beartooth Plateau
near Top of the World, Wyoming. Field
technician Joel Forrest and I were there to conduct snow tracking surveys,
collecting habitat use data that could explain why the red foxes that live
there seem different from those at lower elevations. With kit-rearing dens up to 9,400' (2,820 m) and year-round
occupation of elevations as high as 11,000' (3,300 m), this is the highest
known fox population in North America. It is also distinguished by unique physical and genetic
characteristics: visitors to nearby Yellowstone National Park have long noticed
the lighter blond coat colors and gray underfur of its foxes living at high
elevations compared to the rich red found at lower elevations, and
recent genetic studies have revealed significant differences between foxes
across an elevational gradient within the ecosystem3. Suspecting that behavioral differences could
be contributing to these observations, we wanted to compare genetic and habitat
use patterns high in the Beartooths to those lower down in Yellowstone4,
hoping to identify the mechanisms driving this diversity.
But as we skied closer, I noticed that there was something
odd about this scat. It was not
composed of the fine gray hair and tiny bones one would expect from a predator
of small rodents; instead it prickled with rigid, angular, broken bits of brown
shells. If it wasn't for its small size
and its being found in the wrong season, it could have been mistaken for a
late-summer grizzly bear scat. This fox
scat was packed with crunched whitebark pine nuts.
Later, over beers at the Miners Saloon in Cooke City,
Montana, we reported our discovery to Jesse Logan, an expert on the area's
whitebark ecology as well as its trout fishing and powder skiing.
"I think you are on to something here," Jesse
said, encouraging us to continue documenting this apparently novel behavior,
which was easy to do since the whitebark pine nuts would have a major effect on
fox activity throughout the rest of the winter.
In the following months, we found pine nuts in nearly half
of the 30 scats collected across the territories of multiple foxes, often in
large quantities accounting for most or all of the scat's content. And on several occassions, our snow tracking
surveys even led us deep into the forests, far from the edge territory that foxes
generally prefer, to the raided red squirrel middens that had yielded the
nutritious food. The snow around these
sites, which were usually at the base of a grand old spruce or whitebark, would
be packed down from so many fox tracks and have cone bracts, bark, needles, and
other debris from the excavated midden strewn all over its surface. There would also be short trails leading
away from the midden to smaller packed down rest sites where the fox would
carry a whole cone, pull off its waxy, purple bracts, pluck out the seeds, and
drop the empty husk before returning to the midden for another. Surely this was a more effective way to
obtain calories than by diving through the deep snow after a small, scurrying
vole. And foxes were not the only
carnivores enjoying pine nuts that winter: we also observed several American
marten scats that were obviously loaded with pine nuts.
The following winter, we did not find any pine nuts in the
fox scats collected, but this was expected since we had not heard as many
raucous Clark's nutcrackers that summer, nor did we see the overloaded tree
tops like we had the summer before.
Whitebark pine often exhibit the cyclical reproduction strategy known as
'mast seeding' in which cone production is high in some years and low in others
so as to discourage seed predators from settling in. Although there is substantial variation in
these cycles from whitebark stand to stand given their site-specific
environmental conditions, researchers from the Interagency Grizzly
Bear Study Team conducting cone count transects throughout the ecosystem rated
the first year of this study (2012-2013) a "generally good cone production
year" for the ecosystem, while the following year was rated a
"generally poor cone production year." That second winter, the foxes consumed more
snowshoe hare, which seems like a formidable advisary for a predator that, at
about 9 lbs (4 kg), is no bigger than the average house cat ̶
they may look bigger, but it is all fluff. Nevertheless, we recovered snowshoe hare remains from over 70% of
fox scats that winter, compared to less than 25% the winter before. Also that winter's field tech, Jake Kay, and
I both recorded numerous kill sites while snow tracking, yet neither of us
found any excavated squirrel middens.
Almost everyone who has seen these foxes, or even just their
tracks, in such an extreme environment has wondered, 'what do they eat up here
all winter?' and clearly the answer is whatever they can find. Surviving in the subalpine requires a fair deal
of adaptability, which is something that the red fox, the most widely
distributed terrestrial carnivore in the world, excels at.
So it was not surprising when, at the completion of its
metamorphosis from raw field data to spreadsheets to statistical analyses, we
found significant variance in the food items consumed between the two winters
of this study. But we were surprised
when we likewise analyzed the habitats used between the two winters and again
found statistically significant variance.
During the first winter when whitebark seeds were available, there was a
significant spike in the usage of mature spruce-fir cover types, while the
following winter saw a more even distribution of habitat useage as the spike in
mature spruce-fir leveled off and more mid-successional forest stages were
used. To understand why, we turn to red
squirrel ecology.
Because of interannual variance in whitebark cone production
due to mast seeding, pure whitebark stands are generally considered poor
squirrel habitat since they lack the diversity of food types needed to sustain
squirrels during low cone production years. And where there are no squirrels, there are
no squirrel middens. Spruce-fir cover
types, on the other hand, often have a significant whitebark component in
addition to more consistent but less nutritious food sources. This makes them better squirrel habitat and
the most likely places where whitebark seeds would be available to foxes, thus
explaining the significant spike in spruce-fir habitat use corresponding with
the significant spike in whitebark pine nut consumption.
Ever since we found that first nutty scat high on the
Beartooth Plateau, we were excited since it was, to the best of our knowledge,
the first time foxes were recorded using whitebark pine nuts, adding them to
the long list of animals that directly benefit from whitebark pine. But the combined results of these
statistical analyses are far more profound since they suggest that, beyond
simply using this novel food source, the foxes were actually changing their
habitat use behavior in response to its availability. Perhaps this is a clue pointing to why these high elevation foxes
are distinct; perhaps they have evolved in this landscape where whitebark pine
play such an important role; perhaps the foxes themselves, like the Clark's
nutcrackers, red squirrels, and grizzly bears, play an interactive role in this
particular system. In reality, these
findings do more to raise new questions than they do to answer our original
questions, which may just be the result of good science, but we can be
confident that whitebark pine nuts are an important resource facilitating the
persistence of this fox population in such an extreme environment.
This past spring, I met up with Jesse again at the Miners,
and this time he had questions for me: how cold did it get up there last
winter? what is the snowpack like right now? did you see any blood red brood
trees infested with mountain pine beetles?
When he later returned from a day of bark chipping on the Beartooth
Plateau in the very area where the fox-excavated squirrel middens were located,
his report was grim: wriggling beetle larvae were thriving in those high
forests unaccustomed to the epidemic pest.
Should the Beartooth Plateau experience the dire whitebark declines that
have happened in other parts of the Greater Yellowstone Ecosystem,
the adaptability of its remarkable population of red foxes will be put to the
test.
